Technologies for converting natural gas to liquid fuels may begin with the partial oxidation of hydrocarbonaceous (e.g. methane) or carbonaceous (e.g. coal) materials to a gaseous mixture comprising carbon monoxide and hydrogen, commonly known as synthesis gas, or syngas. An ensuing catalytic process commonly known as the Fischer-Tropsch reaction transforms synthesis gas to a product mixture comprising hydrocarbons. The general chemistry of the Fischer-Tropsch reactions is as follows:nCO+(2n+1)H2→CnH2n+2+nH2O  (1)CO+H2O→CO2+H2  (2)2nCO+(n+1)H2→CnH2n+2+nCO2  (3)
One competing reaction may be the water-gas shift reaction, equation (2), in which at least a portion of the carbon monoxide is consumed in a reaction with at least a portion of the water generated from equation (1), above, to form carbon dioxide (CO2) and hydrogen (H2). The net effect is the consumption of at least some of the water produced in equation (1) and an alteration in the H2:CO ratio.
Catalysts used in the Fischer-Tropsch process vary in composition based upon the product mixture desired and reaction conditions employed but commonly comprise at least one catalytic metal selected from Group VIIIA, preferably Co, Ru, Fe or Ni (according to the Previous IUPAC Form of the Periodic Table of the Elements, as depicted in, for example, the CRC Handbook of Chemistry and Physics, 82nd Edition, 2001–2002, the standard used herein for reference to all element group numbers); optionally, at least one promoter selected from Groups IIIA, IVA, VA, VIA, VITA, and/or VIIIA; optionally, at least one promoter selected from Group IB; and optionally, a structural promoter. The Fischer-Tropsch catalyst may or may not comprise an inorganic refractory support.
The catalytic metal used can influence the nature and composition of the mixture of products and by-products formed. For example, it is well known that iron-based Fischer-Tropsch catalysts have high water gas shift activity and tend to favor reaction (3). By contrast, reactors employing cobalt-based Fischer-Tropsch catalysts tend to favor reaction (1). Fischer-Tropsch reactors utilizing a cobalt-based catalyst can generate significant amounts of water due to the relatively low water gas shift activity of cobalt catalysts. It is known, for example, that the high-temperature, high-pressure steam, typically generated within Fischer-Tropsch reactors employing cobalt-based catalysts can degrade and disintegrate catalyst support particles, causing cobalt to dislodge from the support particles and permitting for the appearance of cobalt subparticles in the product stream. The formation of subparticles that are in the submicron range in a product stream has multiple undesirable repercussions: 1) purification and complete removal of subparticles from the product stream tends to become quite difficult; 2) a reduced lifetime of the catalyst; 3) regeneration of recovered cobalt catalyst tends to be severely hindered; and 4) the loss of costly cobalt metal can represent a significant loss of revenue.
By contrast, iron-based Fischer-Tropsch catalysts have features that make their use potentially attractive in Fischer-Tropsch reactors: iron is considerably less expensive than cobalt; and iron catalysts have inherently high activity in Fischer-Tropsch synthesis. However, its high water-gas shift (WGS) activity makes it a preferred catalyst for Fischer-Tropsch syntheses employing coal-derived syngas (H2:CO 0.5–0.7). Such water-gas shift (WGS) capability enables iron Fischer-Tropsch catalysts to process low H2/CO ratio syngas without an external shift reaction step. Cobalt-based catalysts are preferred in Fischer-Tropsch reactors utilizing natural gas-derived syngas (H2:CO 1.6–2.2). Moreover, iron-based catalysts within the art of Fischer-Tropsch synthesis have been used to make C2–C5 olefins and, less frequently, liquid and waxy paraffinic hydrocarbons. Catalytic compositions for iron-based FT catalysts used to produce C2–C5 olefins, important as chemical precursors for the synthesis of a variety of chemical and petrochemical products, may comprise, for example, a skeletal iron catalyst; for use particularly in slurry phase synthesis processes, as disclosed in U.S. Pat. No. 6,277,895 B1, incorporated herein by reference.
Methods of making iron-based Fischer-Tropsch catalysts are well known to those in the art and may include fusion or precipitation. Preparation and activation of precipitated iron catalysts typically consists of precipitating iron hydroxides and oxides from an aqueous solution; washing, drying and calcining the precipitate; and pretreating the catalyst prior to carrying out the Fischer-Tropsch synthesis reaction. Precipitated iron catalysts having a high support content tend to provide liquid products of high viscosity in slurry-phase reactors; a decidedly undesirable artifact as gas distribution in the slurry is increasingly hindered with increasing viscosity. However, improvements in catalyst attrition resistance can be realized through the use of supported iron catalysts though these catalysts tend to be less active.
Fused iron catalysts are typically prepared by adding promoters to the melted oxide at high temperature. Solid chunks are obtained from the cooled mixture then ground and sized. The specific catalytic activity of fused iron catalysts is generally lower than that of precipitated iron catalysts. Indeed, it has been measured in stirred-tank reactors as half that of precipitated iron catalysts (See Fuel Processing Technology, 1992, Vol. 30, pp. 83–107).
Iron-based Fischer-Tropsch catalysts may be spray-dried and there is some evidence that this can positively affect the catalyst attrition resistance in the catalysts so made. The physical strength of iron Fischer-Tropsch catalysts has recently been improved through spray drying without compromising activity (See for example Ind. Eng. Chem. Res. 2001, vol. 40, pp. 1065–1075 and references cited therein).
Iron-based catalysts may contain at least one, more typically two, promoter elements to assist in CO adsorption and/or iron reduction. With respect to precipitated iron catalysts, potassium is added as a promoter to increase catalytic activity and yield products with higher molecular weight. The effects of potassium on the behavior of iron catalysts may be summarized as follows: (1) a higher α value, resulting in an increase in the average molecular weight of hydrocarbon products; (2) an increased olefin/paraffin ratio in the hydrocarbon product; (3) an increased water gas shift activity; (4) an increased catalyst deactivation rate; and (5) an increased Fischer-Tropsch activity at optimized potassium concentrations.
A copper promoter is typically introduced into iron-based Fischer-Tropsch catalysts to facilitate the reduction of the iron. Copper tends to be more effective than potassium in increasing the rate of the Fischer-Tropsch reaction but tends to attenuate water-gas shift activity. Copper facilitates the reduction of iron and thus decreases the time required to achieve the steady state in FT synthesis. For example, U.S. Pat. No. 5,118,715 relates to pelletized unsupported single phase iron manganese spinels which are dual promoted with both copper and a Group IA or IIA metal useful for selective synthesis of C5+ hydrocarbons from mixtures of CO and H2 in a fixed-bed process.
A structural promoter or binder may be added to an iron catalyst to provide a large surface area for the formation and stabilization of small metal crystallites. For example, silica is sometimes added as a binder to precipitated iron catalysts, especially for those used in fixed-bed reactors; however, its usefulness in slurry bubble-column reactors has not yet been proven. The presence of silica in an iron-based Fischer-Tropsch catalyst may evince any one or more of the following manifestations: 1) A reduction in the concentration of iron in the catalyst requiring the use of greater amounts of catalytic solid which in turn hinders mass transfer; 2) An increase in the concentration of active metal sites through the maintenance of high metal dispersions; 3) An improvement in the aging characteristics of the catalyst.
Iron-based Fischer-Tropsch catalysts may be used in fixed bed or slurry bubble column reactors. Slurry processing provides the ability to more readily remove the heat of reaction, minimizing temperature rise across the reactor and eliminating localized hot spots. As a result of the improved temperature control, yield losses to methane are reduced and catalyst deactivation due to coking is decreased.
To date no one has adequately surmounted the problem of using iron-based Fischer-Tropsch catalysts possessing very low water gas shift activity, such that the carbon dioxide (CO2) selectivity is less than about 18 mol % CO2, while maintaining good CO conversion values, low selectivity for methane and high selectivity and productivity for a hydrocarbon wax product.